tty: Implement lookahead to process XON/XOFF timely

When tty is not read from, XON/XOFF may get stuck into an
intermediate buffer. As those characters are there to do software
flow-control, it is not very useful. In the case where neither end
reads from ttys, the receiving ends might not be able receive the
XOFF characters and just keep sending more data to the opposite
direction. This problem is almost guaranteed to occur with DMA
which sends data in large chunks.

If TTY is slow to process characters, that is, eats less than given
amount in receive_buf, invoke lookahead for the rest of the chars
to process potential XON/XOFF characters.

We need to keep track of how many characters have been processed by the
lookahead to avoid processing the flow control char again on the normal
path. Bookkeeping occurs parallel on two layers (tty_buffer and n_tty)
to avoid passing the lookahead_count through the whole call chain.

When a flow-control char is processed, two things must occur:
  a) it must not be treated as normal char
  b) if not yet processed, flow-control actions need to be taken
The return value of n_tty_receive_char_flow_ctrl() tells caller a), and
b) is kept internal to n_tty_receive_char_flow_ctrl().

If characters were previous looked ahead, __receive_buf() makes two
calls to the appropriate n_tty_receive_buf_* function. First call is
made with lookahead_done=true for the characters that were subject to
lookahead earlier and then with lookahead=false for the new characters.
Either of the calls might be skipped when it has no characters to
handle.

Reported-by: Gilles Buloz <gilles.buloz@kontron.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220606153652.63554-2-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_buffer.c

@@ -5,6 +5,7 @@
 
 #include <linux/types.h>
 #include <linux/errno.h>
+#include <linux/minmax.h>
 #include <linux/tty.h>
 #include <linux/tty_driver.h>
 #include <linux/tty_flip.h>
@@ -104,6 +105,7 @@ static void tty_buffer_reset(struct tty_buffer *p, size_t size)
 	p->size = size;
 	p->next = NULL;
 	p->commit = 0;
+	p->lookahead = 0;
 	p->read = 0;
 	p->flags = 0;
 }
@@ -234,6 +236,7 @@ void tty_buffer_flush(struct tty_struct *tty, struct tty_ldisc *ld)
 		buf->head = next;
 	}
 	buf->head->read = buf->head->commit;
+	buf->head->lookahead = buf->head->read;
 
 	if (ld && ld->ops->flush_buffer)
 		ld->ops->flush_buffer(tty);
@@ -276,13 +279,15 @@ static int __tty_buffer_request_room(struct tty_port *port, size_t size,
 		if (n != NULL) {
 			n->flags = flags;
 			buf->tail = n;
-			/* paired w/ acquire in flush_to_ldisc(); ensures
-			 * flush_to_ldisc() sees buffer data.
+			/*
+			 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
+			 * ensures they see all buffer data.
 			 */
 			smp_store_release(&b->commit, b->used);
-			/* paired w/ acquire in flush_to_ldisc(); ensures the
-			 * latest commit value can be read before the head is
-			 * advanced to the next buffer
+			/*
+			 * Paired w/ acquire in flush_to_ldisc() and lookahead_bufs()
+			 * ensures the latest commit value can be read before the head
+			 * is advanced to the next buffer.
 			 */
 			smp_store_release(&b->next, n);
 		} else if (change)
@@ -459,6 +464,40 @@ int tty_ldisc_receive_buf(struct tty_ldisc *ld, const unsigned char *p,
 }
 EXPORT_SYMBOL_GPL(tty_ldisc_receive_buf);
 
+static void lookahead_bufs(struct tty_port *port, struct tty_buffer *head)
+{
+	head->lookahead = max(head->lookahead, head->read);
+
+	while (head) {
+		struct tty_buffer *next;
+		unsigned char *p, *f = NULL;
+		unsigned int count;
+
+		/*
+		 * Paired w/ release in __tty_buffer_request_room();
+		 * ensures commit value read is not stale if the head
+		 * is advancing to the next buffer.
+		 */
+		next = smp_load_acquire(&head->next);
+		/*
+		 * Paired w/ release in __tty_buffer_request_room() or in
+		 * tty_buffer_flush(); ensures we see the committed buffer data.
+		 */
+		count = smp_load_acquire(&head->commit) - head->lookahead;
+		if (!count) {
+			head = next;
+			continue;
+		}
+
+		p = char_buf_ptr(head, head->lookahead);
+		if (~head->flags & TTYB_NORMAL)
+			f = flag_buf_ptr(head, head->lookahead);
+
+		port->client_ops->lookahead_buf(port, p, f, count);
+		head->lookahead += count;
+	}
+}
+
 static int
 receive_buf(struct tty_port *port, struct tty_buffer *head, int count)
 {
@@ -496,7 +535,7 @@ static void flush_to_ldisc(struct work_struct *work)
 	while (1) {
 		struct tty_buffer *head = buf->head;
 		struct tty_buffer *next;
-		int count;
+		int count, rcvd;
 
 		/* Ldisc or user is trying to gain exclusive access */
 		if (atomic_read(&buf->priority))
@@ -519,10 +558,12 @@ static void flush_to_ldisc(struct work_struct *work)
 			continue;
 		}
 
-		count = receive_buf(port, head, count);
-		if (!count)
+		rcvd = receive_buf(port, head, count);
+		head->read += rcvd;
+		if (rcvd < count)
+			lookahead_bufs(port, head);
+		if (!rcvd)
 			break;
-		head->read += count;
 
 		if (need_resched())
 			cond_resched();